X-ray spot film device

ABSTRACT

In a spot film device, the inner and main carriages for a film cassette are positioned with reversible servo motors. An x-ray grid is mounted on the bottom of the main carriage and a cross grid is mounted on tracks beneath the carriage such that the cross grid may be used or not used for fluoroscopy and both grids may be used for radiography. Field defining masks are also mounted for translating selectively with the carriage. Individual reversible servo motor drives are used to advance and retract the masks and cross grid. Means are provided for absorbing shock or negating drive system momentum when the masks or grid are stopped abruptly. Reduced power is applied to the mask and grid drive motors when the masks and grid reach their stops for holding them against their stops. A motor driven palpator cone, having unique driving and locking means is provided. Means are provided for loading and unloading cassettes from either the front or rear of the spot film device.

BACKGROUND OF THE INVENTION

This invention relates to improvements in an x-ray spot film devicewhich is used in conjunction with an x-ray table to make a selectednumber of radiographic exposures on a single film and to performfluoroscopic examinations.

Conventional spot film devices usually comprise a support or frameextending crosswise over the top of an x-ray table. A main carriage ismounted on the support for being advanced from a rearward parked postionto a frontward radiographic position wherein a cassette carried by thecarriage is disposed in alignment with an x-ray beam that is projectedthrough a patient from an x-ray source in the table. The cassette ismounted in a tray supported on an inner carriage which is translatablecrosswise of the main carriage so that the center of the x-ray beam maybe made coincident with the area on the film cassette on which a spotfilm exposure is desired. The area is further defined by superimposablemasks.

As is well known, spot film devices are also provided with afluoroscopic device which permits an examining radiologist to visualizeanatomy of interest and to make one or more radiographs in a choice ofsizes by projecting the cassette forward and shifting it and the masksto obtain the desired sequence of radiographs. The fluoroscopic deviceon the spot film device is aligned with the x-ray source in the table,and the film cassette is, of course, retracted from the beam duringfluoroscopy.

When a fluoroscopic view of interest is observed, the film cassette mustbe projected into the x-ray beam path rapidly and one or more exposuresmust be taken while the fluoroscopically observed condition persists. Inprior art spot film device, the main carriage which supports the innercarriage on which the film cassette is mounted is usually drivenrearwardly to parked position with a unidirectional motor that moves itslowly and loads a return spring at the same time. The carriage islatched in parked position and when the latch is released the carriageis advanced rapidly under the influence of the spring and haltedabruptly in the radiographic position. Rapid movement ad abrupt halt ofthe carriage and the masks results in considerable noise, shock andvibration that necessitate use of shock absorbing devices such as dashpots to reduce these ill effects. One problem with this system is thatthe main carriage must be restored to a rearward position after eachexposure, to reload the spring, after which the carriage must beprojected forwardly again to make the next exposure.

Some prior spot film devices have a set of tracks for the main carriage.After each exposure, the carriage is returned rearwardly and shifted todifferent tracks, similar to railrod car switching. Then the carriage isdriven forwardly and it arrives in the proper position for the nextexposure to be made. This is a relatively slow method and it requires alarger and complicated mechanism which has many moving parts.

In prior art spot film devices two x-ray field defining masks havingelongated rectangular apertures disposed at right angles to each otherare used. One of the masks is moved manually into the x-ray beam whentwo adjacent cross-wise exposures are to be made on the film. The othermask is usually carried on the palpator cone device so it and thepalpator cone are disposed in the beam when two adjacent exposureslengthwise of the film are desired. Both masks and the cone are in thebeam when quadrants or four exposures on one film are desired. Manualmovement of the masks, in accordance with the prior art, is distractingto the radiologist and it tends to slow down the procedure undesirably.

Some prior art spot film devices dispose an x-ray anti-scatter gridpermanently in the x-ray beam path. As is well known, a grid iscomprised of a large number of lead strips disposed edgewise of thex-ray beam and in parallelism with each other or at predetermined anglesin focused grids. A grid permits passage of x-rays which emanate fromthe focal spot of the source in a direct or straight line to theradiographic film or fluoroscopic device. Rays which are scattered bythe equipment and by the intervening body being examined are interceptedby the lead strips and are largely prevented from reaching the film orfluoroscopic device where they may cause undesirable fogging of theimage and poor sharpness.

Fluoroscopic procedures are preferably conducted with a low ratio gridor no grid. Radiographic procedures, on the other hand, are preferablyconducted with a high ratio grid. In prior art apparatus where only onegrid is available, a low ratio grid is usually installed since it can beused for fluoroscopy and radiography even though sharpness of the filmimage is not as good as it would be if a high ratio grid were used forradiography. The high ratio grid cannot be the sole grid used forradiography and fluoroscopy since it would reduce image intensity orbrightness too much for fluoroscopy.

Another component of a spot film device to which consideration will begiven is the palpator. The palpator comprises an x-ray permeable conewhich may be advanced into the x-ray beam path during fluoroscopy. Thecone may be brought down by moving the whole spot film devicedownwardly, to depress the patient so that the movements of an x-rayopaque material which may have been ingested by the patient can bevisualized on the fluoroscopic device. On some occasions, theradiologist moves the spot film device laterally or frontwardly andrearwardly while the cone is impressed in the patient's body. Thisrequires the radiologist, after having moved the cone carriage forwardlyas in prior art spot film devices, to lock the cone so it will not slipon the spot film device while the device is being manually oscillated toeffect palpation. The need for the radiologist to devote attention tomoving the palpator manually and to locking and unlocking it is alsodistracting and slows down the procedure.

SUMMARY OF THE INVENTION

General objects of the present invention are to overcome the abovedefects in prior art spot film devices, to provide some new features ofconstruction and operation for expediting radiographic spot film andfluoroscopic procedures, to minimize operational noise and vibration,and to simplify construction and operation of a spot film device.

Another object is to provide means for advancing the main carriage of aspot film device from parked position to radiographic position andretracting it to parked position rapidly and quietly with reversibleservo motors that drive the main carriage and the inner carriagesupported thereon in both directions to avoid the need for returning themain carriage to parked position for recocking a driving spring orswitching tracks after each spot film is exposed as was required inprior art apparatus.

Still another object of the invention is to provide filed defining mask,an x-ray grid and a palpator cone device which may be translatedindependently of each other.

A general object is to provide stops which arrest travel of the masksand grid in the proper positions for their use and, further, to providemeans for negating system inertia when travel is arrested and to providefor constraining the masks and grids against their stops, such as bykeeping their drive motors energized with reduced electric power.

A further object is to provide a drive system for advancing andretracting the masks which uses a reversible servo motor for each maskand a belt which is fastened to the mask with springs that can bestretched slightly to absorb shock, incidental to rapid movement andstopping of the masks and, further, to reduce the electric powersupplied to the reversible motors after the mask has reached its finalposition so that the motor, with reduced power thereon, will tend tomaintain the mask in the correct final position regardless of theangular attitude of the spot film device.

Another object is to provide an independently translatable anti-scatterx-ray grid which may be selectively inserted in or retracted from thex-ray beam during fluoroscopy and during radiography, using a drivesystem that is similar to that just outlined in respect to the masks.

Yet another object is to provide a radiographic grid which is moved intoand out of the x-ray beam path with the main carriage and which has ahigher ratio and different orientation than the fluoroscopic grid sothat, during radiography, the two grids may be used in their crosswiseorientation to produce the anti-scattering effect of a high ratio grid.

Another object is to provide a palpator device which is independentlymovable and is advanceable and retractable into and out of the x-raybeam path using a reversible servo motor, a belt drive incorporatingelastic means such as the springs mentioned above, and which uses aunique latch for holding the palpator in advanced position on the spotfilm device and which is releasable automatically in response to theservo motor being commanded to reverse its direction under full power.

Yet another object is to provide light weight masks which can beadvanced and retracted at high speed with the application of littlepower and are characterized by laminations of heavy lead sheets andlight weight stronger metal sheets.

How the foregoing and other more specific objects of the invention areachieved will be evident in the following more detailed description of apreferred embodiment of the invention which will now be set forth inreference to the drawings.

DESCRIPTIONS OF THE DRAWINGS

FIG. 1 is a perspective view of a typical diagnostic x-ray table withthe new spot film device installed;

FIG. 2 is a plan view of the spot film device shown in the precedingfigure from which the x-ray image intensifier or fluoroscopic device hasbeem removed;

FIG. 3 is a partial section of the margin or a door opening taken alongthe line 3--3 in FIG. 2 to show the elements of a tape switch that actsto prevent operation of the mechanism in the spot film device if a handextends into the doorway during insertion or withdrawal of a filmcassette;

FIG. 4 is a plan view of the upper frame or support of the spot filmdevice which exhibits the main carriage and the inner film cassette traycarriage along with motor operated positioning mechanisms for thecarriages;

FIG. 5 shows a section of a door taken along the line 5--5 in FIG. 4which door is for inserting and withdrawing a film cassette at the frontend of the spot film device;

FIG. 6 is a plan view of a latch mechanism for constraining the maincarriage in its most advanced position to permit inserting andwithdrawing a film cassette;

FIG. 7 is an elevation view of the latch looking in the direction of thearrows 7--7 in FIG. 6;

FIG. 8 is a plan view of the main carriage and inner carriage isolatedfrom the spot film device;

FIG. 9 is a side elevation view of the apparatus shown in FIG. 8 lookingin the direction of the arrows 9--9;

FIG. 10 is a plan view of a pan-like support and mechanism thereon whichpan, in the assembled spot film device, is mounted to the bottom of theframe shown in FIG. 4;

FIG. 11 is a fragmentary view of a member that carries a palpator conewhich is associated with a latch that is self-locking and automaticallyreleasable;

FIG. 12 is a side view of the main frame shown in FIG. 4 with a partbroken away to show the drive mechanism for the main carriage;

FIG. 13 is a partial section taken along the lines 13--13 in FIG. 10, toexhibit one of the tracks on which the field defining masks and an x-raygrid are guided;

FIG. 14 is a front end view of the frame, shown in FIG. 4, to which apalpator cone is attached;

FIG. 15 is a plan view of one of the apertured x-ray field definingmasks;

FIG. 16 is a section of the mask taken along the line 16--16 in FIG. 15;

FIG. 17, comprised of parts 17A-17E, shows diagramatrically therelationship between the film cassette and the masks for obtainingexposures on front and rear halves of the film, on the left and righthalves, and one of the quarter-sections of the film;

FIGS. 18 and 19 are diagrams of control systems.

DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 is a perspective view of a typical diagnostic x-ray tableincorporating the improved spot film device. The table comprises a body10 in which there is an x-ray source, not visible. When energized, thex-ray source projects a collimated x-ray beam through table top 11 onwhich a patient undergoing x-ray examination may be reposed. The top hasa foot rest 12 for supporting the patient in an upright position whenthe table body 10 is angulated clockwise from the position in which itis shown. Table body 10 is supported from a floor stand 13 with respectto which body 10 may be angulated and translated to clear the floor witha mechanism and driving means that are not shown since they areconventional. A locking mechanism 15 holds foot rest 12 to the table top11 in whatever position of adjustment is desired.

The x-ray source, not visible, is mounted on a carriage which is alsonot visible but is located within table body 10 and is adapted for beingtranslated in opposite directions longitudinally of the body or, inother words, lengthwise of the patient. Extending upwardly from thecarriage at the rear of the table is a column 16 which may be extendedand contracted in the vertical direction.

The improved spot film device is generally designated by the referencenumeral 17. It is supported on column 16 by means of a bearing support18 that cooperates with a pair of tracks, such as the one marked 19, toenable the spot film device to be shifted manually to a limited extentcrosswise of the table top and parked toward the rear.

Mounted to the top of the spot film device 17 and near its front is afluoroscopic device which is generally designated by the referencenumeral 20. In modern practice it is customary to use an x-ray imageintensifier for fluoroscopy and the use of such a device is assumed inthis case. A television camera, not shown, mounted within a housing 21is used to display the x-ray image, obtained during a fluoroscopicprocedure, on a television monitor which is not shown but is well knownto those who are skilled in the art.

The control console for operating the spot film device is located at itsfront end and is marked with the numeral 22. The spot film device has afront opening 23 for inserting and withdrawing a film cassette at thefront of the table. At the top of the device and behind imageintensifier housing 20 there is another opening 24 which provides theoption of inserting and withdrawing a film cassette at the rear.

Attention is now invited to FIG. 4 which shows the upper frame 30 of thespot film device and the mechanism carried thereby. These componentswill be described in detail soon. Before proceeding, however, attentionis invited to FIG. 10 which shows some additional mechanism that iscarried by a support which, for convenience, is called a pan 31. Itshould be understood at this juncture that pan 31 is assembled to thebottom of frame 30 in FIG. 4 so that pan and frame are substantiallycongruent. The two subassemblies of FIGS. 4 and 10 constitute theprincipal components of the spot film device.

Referring to FIG. 4, frame 30 is essentially a one piece casting of alight weight metal such as magnesium alloy. Frame 30 has a front end 32,a rear end 33 and sides 34 and 35. A pair of parallel tracks 36 and 37are mounted within frame 30. A first carriage, herein called a maincarriage 38 runs on tracks 36 and 37. Main carriage 38 may be retractedto parked position which is somewhat rearward of the position in whichit is shown. It may also be advanced frontwardly to where it becomesaligned with central ray of an x-ray beam projecting upwardly from thex-ray source in the table body. The location of the central ray ismarked 39 in FIG. 4.

Main carriage 38 has sides 40 and 41 which may be provided with rollersand sleeve bearings, not visible, or other linear bearing means forcooperating with tracks 36 and 37. Mounted on main carriage 38 is asecond carriage, herein called to the inner carriage 42. Inner carriage42 is movable crosswise of main carriage 38 between the limitsdetermined by sides 40 and 41 of the main carriage.

The inner carriage translates on rollers, not visible, which extend fromside 43 and engage tracks in side 44 of the main carriage. The innercarriage is further guided on linear bearings that cooperate with a rod45 which is attached at opposite ends 46 and 47 to opposite sides 40 and41 of main carriage 38. One of the linear bearings which is located inthe portion of inner carriage marked 48 in FIG. 4 may be seen in FIG. 8where the bearing is marked 49. The roller bearings 50 and 51 for innercarriage 42 are also visible in FIG. 8.

The inner carriage 42 is provided with a tray for holding a filmcassette which will be discussed in more detail later. As in many priorart spot film devices, the film cassette is carried forward by the maincarriage and shifted crosswise thereof to make the central x-ray 39coincide with an area of the film on which an x-ray exposure is to bemade.

One distinguishing feature of the new design is the manner in which themain carriage 38 is advanced to radiographic position in coincidencewith central ray 39 and retracted to parked position toward the rear offrame 30. In a commonly used prior art design, the main carriage isretracted to parked position with a motor drive while loading a springat the same time. When the carriage is unlatched, it is projectedforwardly to radiographic position with energy stored in the spring. Inother prior designs, the main carriage is retracted, shifted todifferent tracks and advanced again, using a motor to drive in bothdirections.

In the new design shown in FIG. 4, the main carriage is advanced andretracted in both directions exclusively with the power obtained from areversible servo motor 55 that is mounted to the end member 33 of frame30. The motor drives a belt 56 which is coupled to the main carriage andtranslates it either forwardly or rearwardly, depending on therotational direction of the motor. In FIG. 12, frame 30 is broken awayto show a profile of the main carriage drive system. Considering FIGS. 4and 12 together, one may see that belt 56 is essentially an open loopwhose ends 57 and 58 are clamped to posts 59 and 60 which are fastenedto main carriage 38. Belt 56 runs on a drive pulley 61 and an idlerpulley 62 which is spaced from the drive pulley by a distance that issubstantially coextensive with the travel distance of main carriage 38.In FIG. 12, it is apparent how a row of studs such as the one marked 63support the track 37 on which the linear bearings 64 and 65 of the maincarriage 38 glide. Linear bearings 64 and 65 are closely fit to track 37so the carriage follows a perfectly straight line when it is advancedand retracted.

Use of a reversible motor, pulleys and a belt to translate the maincarriage is to be considered illustrative rather than limiting. It is,for example, within the scope of the invention to use a motor drivenlead screw or other suitable means to translate the carriage.

Referring again to FIG. 4, a potentiometer 66 is provided for producinga signal which corresponds with the position of main carriage 38. Motorpulley 68 drives a belt 72 which drives a pulley 61. There is a commonjournaled shaft for pulley 67, pulling 61 and a pinion 69. Pinion 69drives a reducing gear 70 which is on the same shaft as is potentiometer66 so potentiometer 66 is driven to angular positions corresponding withthe linear distance travelled by main carriage 38. Commands for runningmotor 55 to advance and retract main carriage 38 and signals derivedfrom potentiometer 66 for sensing the position of the carriage arecoordinated in a program controller not shown, and which is remotelylocated together with a servo amplifier to provide a position servosystem.

The drive mechanism for shifting inner carriage 42 alternately on maincarriage 38 as required for exposing selected areas on the film will nowbe described primarily in reference to FIG. 4.

Inner carriage 42 is translated selectively with a reversible servo gearmotor 75 located near the rear end of main frame 30. The motor ismounted on a bracket 76 which is fastened to rear end 33 of the frame.Mounted directly on the shaft of gear motor 75 is a cable sheave 77which has grooves for accommodating a cable whose opposite runs aremarked 78 and 79 at the rear of frame 30. The cable makes several turnsaround sheave 77. Cable run 79 extends around an idler pulley 80 fromwhich it runs over a pulley 81 that is journaled for rotation on maincarriage 38. The cable continues around a pulley 82 which is journaledfor rotation on inner carriage 42. The cable then continues around apulley 83 on the main carriage after which it extends around a groovedpin 84, on which there could be another pulley, after which the cablecontinues to a tension adjusting screw 85 to which it is attached. Cablerun 78 similarly extends around an idler pulley 86 and continues arounda pulley 87 on main carriage 38 pulley 88 on inner carriage 42, pulley89 on the main carriage and then around pin 90 after which it loops backto a tension adjusting screw 91 to which it is attached.

When servo motor 75 is commanded by a second position servo system, notshown, to rotate in one direction such as to cause sheave 77 to rotateclockwise as viewed from the rear end, cable run 78 will be placed intension and will tend to wind on sheave 77. This will shorten run 78 ofthe cable and will result in inner carriage 42 being pulled toward theobserver in FIG. 4. At the same time, cable run 79 will unwind fromsheave 77 correspondingly so that the loop of cable passing aroundpulley 82 on the inner carriage will be extended in response to theinner carriage 42 being pulled toward the observer.

A unique feature of the system which has just been described is that useof a reversible servo motor 75 permits translating the inner carriage 42while the outer carriage is in transit toward radiographic position inwhich case the cassette will be in proper position for conductingradiography by the time it arrives in its advanced position. Moreover,the inner carriage and a cassette thereon may be translated while theouter carriage is in motion to the desired position for removing acassette from the rear of the spot film device. In addition the systempermits multiple exposures to be made without the need of the carriageto return to the rear film loading position between exposures.

Referring to FIG. 4, one may see that there is an x-ray anti-scattergrid 92 mounted on the bottom of the inner carriage. A portion of thetop face of the grid is broken away to show the orientation of its leadx-ray attenuating strips 93 which appear edgewise and extend along thedirection in which the main carriage moves. This grid is, of course,only disposed in the x-ray beam for radiography with the film cassette.Later there will be discussion of how grid 92 cooperates with anothergrid to enable cross-grid radiography with reduced x-ray scattering andfilm fogging. Cross-grid radiography is discussed in the magazineRADIOLOGY, Volume 115, pages 732-733, June, 1975, in an article bySkucas et al. entitled "New Grid Design for a Fluoroscopic Spot FilmDevice".

In FIG. 8, the inner carriage 42 is shown isolated from main carriage 38to facilitate a more detailed description of the inner carriage. Mountedon the inner carriage 42 is a cassette tray 95 comprised of framemembers 96, 97 and 98 on which a conventional flat cassette, not shown,may be supported. The frame members define a rectangular window 99 forthe x-ray beam which projects upwardly from the source in the table bodyto expose a film in the cassette after having passed through the tabletop, a patient and grid 92. Cassette tray 95 is movable in parallel withside members 100 and 101 typically with linear bearings such as 125adapted to slide on a rod 124. Means are provided for advancing cassettetray 95 to the position on inner carriage 42 in which it is shown inFIG. 8 to enable insertion and withdrawal of a cassette from the frontend of the spot film device. When a cassette is in tray 95 and is readyfor radiographic purposes, cassette tray 95 is normally shifted to aposition more to the right in inner carriage 42 than the position inwhich it is depicted in FIG. 8. When main carriage 38 is driven to itsmost advanced cassette loading and unloading position at the front ofthe frame 30 in FIG. 4, the tip 102 of a plunger rod 103 strikes thefront end 32 of frame 30, causing the rod to be forced rearwardly asshown in FIG. 8. The rear end of rod 103 has an arm 104 on which thereis a pivot pin 105. An articulated linkage comprised of links 106 and107 makes a connection between pin 105 and a sliding pin 108 whichextends from near the end of link 107. Link 107 swings on pivot pin 112.When the cassette tray 95 is retracted, sliding pin 108 is positioned atend 109 of a slot 110 which is in flange 111 extending from the cassettetray. When link 107 pivots on a shaft 112 carried by a bracket 113, pin108 slides in slot 110 and rotation of link 107 forces tray 95 forwardlyto the cassette loading and unloading position in which it is shown inFIG. 8.

As can be seen in FIG. 9, plunger rod 103 has a shoulder 114 on it. Rod103 slides through a bearing 115. A coil spring 116 is captured betweenshoulder 114 and the stationary bearing 115. This spring is compressedby the plunger rod when the carriage is driven to its most forwardposition. When the carriage is driven rearwardly, after having had anexposed film cassette removed and unexposed film cassette inserted intray 95, spring 116 restores plunger rod 103 to its extended position asit appears in FIG. 4.

Extending from cassette tray 95 are a pair of arms 117 and 118. Arm 118is terminated with a door striker 119 which, as can be seen in FIG. 9,has a beveled surface 120 constituting its tip. When the main carriage38 moves to its most frontward position on frame 30 as shown in FIG. 4,beveled tip 120 strikes a door 121 and causes it to open so that a filmcassette which has been advanced enough to extend through the door byaction of plunger rod 103 can be grasped with fingers for removal. Thedoor, of course, remains open until the main carriage is retracted.

Door 121 is shown in section in FIG. 5 and is seen to comprise atriangularly formed stiff metal sheet 122 which has a lead strip 123backing it for the purpose of attenuating stray x-radiation. The door ismaintained in the closed position in which it is shown in FIG. 5 with atorsion spring 127.

After having advanced main carriage 38 to its most forward position forloading or unloading a film cassette, servo motor 55 is de-energized. Alatch is provided for preventing carriage 38 from being propelledrearwardly by the energy stored in compressed coil spring 116 whichsurrounds plunger rod 103. The latch is shown in FIGS. 6 and 7. Itcomprises a bell crank 130 carried by a pivot shaft 131 that issupported on the leg 132 of a bracket 133 which has another leg 134.Bracket 133 is fastened on the inner side of main frame side member 34along the path of travel of main carriage 38. Bell crank 130 has a camroller 135 and it is biased to the position in which it is shown in FIG.7 with a spring 136. In FIG. 7, a fragment 137 of a cam which has twobeveled spaces 138 and 139 is shown. This cam is mounted on the bottomof side 40 of carriage 38. When the cam translates with carriage 38 toits cassette loading and unloading position, its beveled surface 138strikes roller 135 and rocks the bell crank such that roller 135 issubsequently engaged by beveled surface 139 of the cam. Carriage 38 isthereby held forwardly or to the right in FIG. 7 but it can be retractedby operating main carriage drive motor 55 in its reverse direction suchthat it provides sufficient power to overcome the camming force ofspring 136 in which case the carriage 38 is driven rearwardly by themotor.

The unique manner in which the x-ray image field defining masks areconstructed and driven will now be described in reference to FIG. 10. Asmentioned earlier, the spot film device is made in two major assemblies,one of which comprises support means called a pan 31 which is mounted tothe bottom of main frame 30. The pan may be mounted with screws insertedthrough holes such as those marked 145 and 146 in the margin of the panin FIG. 10.

In FIG. 10 the x-ray beam as modulated by the image would be coming upfrom the bottom of the pan when the spot film device is extending overthe x-ray table top, the point at which the central ray of the beamcomes up is marked 153.

In FIG. 10, there is a lower planar mask 147 which has a rectangularaperture 148 for defining an x-ray field. The leading edge of mask 147is marked 149 and its trailing edge is marked 150. There is also anupper mask 151 which has an aperture 152 for defining an x-ray field.Rectangular aperture 152 has the same shape and size as rectangularaperture 148 in mask 147 but the long dimension of the aperture 152 inmask 151 is perpendicular to the long dimension of aperture 148 in mask147.

Between upper mask 151 and lower mask 147, there is another planaranti-scatter x-ray grid 154 which has a corner broken away to show thatits lead strips 155 are directed crosswise of the lead strips 93 of grid92 in FIG. 4. Grid 154 is selectively movable in and out of the x-raybeam as will be explained later. The leading edge of grid 154 is marked156 and its trailing edge is marked 157.

Masks 147 and 151 and grid 154 are mounted for translating on a pair oftracks 158 and 159 which are arranged in parallelism near oppositemargins of pan 31. Grid 154 is mounted on an apertured metal plate 169which has co-planar tongues 161 and 162 extending from it on the sameedge. It also has tongues 163 and 164 extending from its opposite edge.Typical of tongues 163 and 164, their margins 165 are bent upwardly toform an L-shaped cross-section. Referring to FIG. 13 where a section oftracks 158 are shown, it will be seen that the track has a guide slot166 which is also L-shaped and is complemented by tongues 163 and 164.One of the tongues is marked 165 in FIG. 13 as it is in FIG. 11. Use ofthe angularly bent tongues assures that the grid will not shiftlaterally when sliding in the grooves of the tracks.

The side of mask 147 also has tongues or extensions 167 and 168 whichhave L-shaped cross sections. A cross-section of track 159 is not shownbut it will be understood that it has complementary L-shaped grooves inwhich the L-shaped tongues of the mask glide. Extensions 169 and 170from the other side of mask 147 are planar and it will be seen in FIG.13 that the planar tongue 169 slides in a corresponding planar groove intrack 158. Mask 151 also has similar tongues which function in themanner just described in respect to mask 147. A more detaileddescription of the structure of the masks will be given later inreference to FIG. 15.

The manner in which masks 147 and 151 and cross-grid 154 are advanced toradiographic position and retracted to parked or inactive position, willnow be described in reference to FIG. 10, primarily. There is areversible servo motor for the lower and upper masks and the grid,respectively. Motor 175 drives or translates upper mask 151. Motor 176drives lower mask 147. Motor 177 drives grid 154.

Consider the upper mask drive system first. It comprises a sprocket-likepulley 178 on the shaft of motor 175. Situated near the front end of pan31 is an idler pulley 179 which is mounted for rotation on a bracket180. Running on drive pulley 178 and idler pulley 179 is an open loopbelt which has opposite ends 181 and 182 that are connected to theleading and trailing edges of mask 151 with coil springs 183 and 184,respectively. Hence, operation of motor 175 in one direction willretract mask 151 to parked position and rotation of the motor in anopposite direction will advance mask 151 to the front of the spot filmdevice, that is, to the left in FIG. 10, for use in fluoroscopy ifdesired and for cross-grid radiography.

Disposed in the path of the masks and the grid at the front end of pan31 is a limit switch assembly 186 which has several plungers indifferent planes. Two of the plungers which are superimposed over theothers are marked 187 and 188. Depression of the plungers operates limitswitches, not visible, in assembly 186. A stop 189 is also provided foraccurately determining the maximum travel of the masks and grid. Stop189 may have a metal face and a resilient backing to minimize noise andshock.

By way of example and not limitation, in a commercial design the leadingedge, such as edge 190 of mask 151, strikes plunger 187 and operates alimit switch about 0.2 of an inch before the mask hits stop 189.Substantially full rated electric power is applied to servo motor 175while it is driving mask toward the stop 189. When leading edge 190 ofmask 151 strikes the plunger 187, however, the accompanying operation ofa limit switch results in the power applied to servo motor 175 beingreduced substantially. Typically, in a commercial embodiment, the poweron servo motor 175 is reduced to about one-tenth of power that isapplied when the motor is driving the mask through most of its travel.The reduced power on motor 175 is continually applied so that theleading edge of the mask is held firmly against stop 189. Hence, evenwhen the spot film device is tilted with the x-ray table, the mask isheld firmly against the stop by the motor in opposition to the force ofgravity acting on the mask. The circuitry which responds to actuation ofa limit switch by reducing power on the servo motor can be devisedeasily so it is not described for the sake of brevity.

When a mask is driven against stop 189 incidental to the momentum of thesystem, the elastic coil springs 183 and 184 in the drive belt arestretched by a small amount such that they contribute further toabsorbing shock but loading of the springs does not result in any recoilsince the servo motor remains energized at a reduced power level. Thus,the system has the merits of smooth and quite operation.

Coil springs 183 and 184 do not necessarily have to be directly couplingthe ends of the belt and the masks. They could be located at otherpoints in the belt as long as they do not run over the pulley. Anelastic section might also be inserted in the belt or, in some cases, abelt with just the right elastic properties might be used. In acommercial embodiment, a rubber belt having teeth on its running surfacewas used and pulleys 178 and 179 have corresponding sprocket teeth toassure positive drive and to assure that shock will be absorbed byslightly stretching the springs which couple the belt ends to the mask.

It should be noted that leading edge 190 of mask 151 has a notch 192which allows the advancing mask to clear and not actuate plunger 188while desirably actuating plunger 187 of the limit switch assembly.

When the mask 151 is driven to its most retracted or parked position, itencounters another limit switch assembly 195 which has a set of plungersincluding those marked 196 and 197. At the end of its retracted travel,the rear edge 191 of mask 151 strikes plunger 196 to actuate a limitswitch which results in reducing the power applied to servo motor 175.Again, the belt coil spring that is in tension when the edge 191 strikesa stop 198 is stretched or loaded slightly to absorb shock. Edge 191 ofmask 151 has a notch 199 so that striking of plunger 197 can be avoidedwhen plunger 196 is to be depressed.

The drive system for lower mask 147 is similar to that for mask 151which has just been described. Thus, there is a reversible servo motor176 for driving lower mask 147. There is a pulley 200 on the motor shaftand an idler pulley 201 spaced from it and there is a belt loop whoseupper run is separated into two parts 202 and 203 to enable coupling theends of the belt to the mask with elastic means such as springs 204 and205. One edge of lower mask 147 has a notch 206 so it will avoidactuating plunger 188 while actuating plunger 187 during advancementtoward stop 189 and it has a notch 207 in its rear edge so it will avoidactuating plunger 197 while operating plunger 196 before striking stop198 when it is being retracted. As in respect to upper mask 151, whenthe lower mask 147 reaches either limit of its travel, reduced power isapplied to servo motor 176 and the mask is held securely against a stop.

X-ray grid 154 is driven in a fashion similar to that of the masks withits individual reversible servo drive motor 177. Thus, the drive shaftof motor 177 has a pulley 210 which drives a belt 211 over a remotelyspaced idler pulley 212. As in the case of the masks, belt ends areattached to opposite edges of the grid with coil springs 213 and 214.

The leading edge 215 and the trailing edge 216 of grid 154 operate limitswitches in assemblies 186 and 195, respectively, near the end of theirtravel in the advanced or retracted direction and cause reduced power tobe applied to servo motor 177. As in the case of the masks, full poweris immediately applied to the servo motor when operation in the reversedirection is commanded so that the grid will travel over most of thedistance at top speed.

From the description thus far, those skilled in the art will recognizethat the radiologist has the option of advancing np masks, one mask orboth masks for radiography and the further option of either advancing ornot advancing the grid into the x-ray beam for fluoroscopy. The optionof not advancing grid 154 or advancing it to perform cross gridradiography in cooperation with grid 92 on the bottom of the carriage isalso provided.

Recapitulating, fundamental concepts of the mask palpator and grid drivesystems are to have the masks, palpator and grids come to a definitestopping position at a reasonable speed and to remain in that positionuntil the servo motor is commanded to drive in the reverse direction.This is implemented in the preferred embodiment by reducing the power onthe drive motor an instant before the mask, grid or palpator hits astop. The motor and drive system has inertia, however, which wouldresult in a sharp impact but this is negated by permitting the motor toovertravel and load a spring which holds the movable devices against thestop. Overtravel might also be obtained by other means such as bydriving through a slip clutch, not shown, instead of using an elasticbelt or connecting a belt to the movable devices with springs. Reducedpower on the drive motors can still be used advantageously to hold themovable devices against the stop. If no springs or other elastic meansare used, the shock on the system may be so great that the drive motorshafts or other components such as pins or keys might shear.

Attention is now invited to FIGS. 10 and 14, primarily, for adescription of the palpator means and its driving system. FIG. 14 showsthe upper or main frame 30 of the spot film device with the lowersupport means or pan 31 fastened to the bottom of the main frame. Thepalpator comprises a centrally apertured plate 220 which has anupstanding margin 221 and an inwardly bent portion 222 on one side. Italso has an inwardly bent portion 223 on its other side. Inwardly bentportions 222 and 223 serve as bearings on which the palpator may beadvanced to fluoroscopic position and retracted to parked or inactiveposition. The palpator is provided with a support plate 224 on which anx-ray transparent cone 225 is mounted for use in a well-known manner.

In FIG. 10, the leading and trailing edges 226 and 227 of plate 220appear as dashed lines while the over-hanging margins 222 and 223 appearas solid lines. The palpator assembly is advanced and retracted with itsindividual reversible servo motor 246 located in the rear portion of pan31 as are the other servo motors 175, 176, and 177 for the masks andgrid. The drive system for the palpator is similar to that of the masksand grid. It comprises a pulley 247 on the shaft of motor 246 and anidler pulley 228 on which a belt having ends 229 and 230 runs. The beltends are coupled to the palpator with springs 231 and 232. The palpatordrive system employes the feature of reducing power on its servo motor246 when the palpator reaches either of its travel limits.

When the spot film device is lowered toward the patient to enableapplying a force on the patient with cone 225, the radiologist sometimesdesires to move the palpator through short distances crosswise of thetable in order to urge portions of the anatomy in the desired manner fora fluoroscopic study. This is usually done by shifting the whole spotfilm device, basically the upper frame 30, on its supporting tower atthe back of the x-ray table. Hence, the palpator must be locked againsttravel on the spot film device during this procedure.

A unique locking device which requires no conscious manual activity norsolenoid operator to release it is provided. Refer to FIG. 11. When thepalpator is driven as far as it will go to the left as it appears inthat figure it will hit a stop, not shown, which will prevent it fromshifting any further in that direction. It could, however, shift to theright undesirably but for the latch device which is shown in FIG. 11. Abracket 235 shown attached to the palpator in FIG. 11 serves as a meansfor coupling the end 230 of the belt to the palpator with a spring 231.The palpator has another bracket 236 mounted on it. Mounted on bracket236 with a pivot pin 237 is a latch arm 238. The pivotal latch arm has abeveled cam surface 239 and a hook portion 240. Bracket 236 and latcharm 238 travel with the palpator. A first spring 241 is coupled betweena pin 242 on the latch arm and a pin 243 on the bracket. Spring 241normally tends to rotate latch arm 238 in the clockwise direction asshown in FIG. 11. The hooked end 240 of the latch arm is adapted toengage a stationary pin 244 which is located appropriately along thepath of travel of the palpator. Assume that the palpator moves in thedirection of arrow marked 245 in FIG. 11 when the palpator is beingdriven to its active position wherein locking it is desired. When itreaches active position, cam surface 239 on latch arm 238 will strikestationary pin 244 to overcome the force of spring 241 so the hook 240will engage with pin 244 and prevent the palpator from being moved in adirection that is opposite to that of arrow 245. Upon this event, spring231, which couples the belt end 230 to the palpator will be slightlystressed and reduced power may be applied to the palpator drive motor.

Now assume that the operator desires to restore the palpator to itsretracted or parked position, which means moving it to the left as itappears in FIG. 11. All that is necessary is to command palpator servomotor 246 to drive in the reverse direction in which case belt end 229is placed in tension and coupling spring 232 is stressed. Couplingspring 232 is hooked onto pin 242 of the latch arm so that when it isput in tension, latch arm 238 is caused to rotate through a small anglein a counterclockwise direction to release the hook 240 from stationarypin 244. In other words, the tension on the belt develops a force thatis counter to the biasing force of first spring 241. The fulcrum betweenpivot pin 237 and spring attaching pin 242 is the same for first spring241 and the second spring 232 so it should be evident that the secondspring 232 which is in series with the belt must be the stiffer of thetwo springs or it would stretch when the belt is tensioned withoutreleasing the latch arm. Thus, it will be seen that simply operatingpalpator drive motor 246 to put spring 232 in tension will overcome thelatchin force of spring 241 and will release the latch.

The unique construction of the masks will now be described withreference to FIGS. 15 and 16 which show one of the masks 151. The maskis made in a laminar fashion so it will have light weight strength andyet have sufficient x-ray attenuation in the proper areas to cut offundesired portions of the x-ray beam and define a sharp field. Referringto FIG. 16, the bottom plate which must span a substantial distancebetween the tracks on which the masks slide, is made of a stiff sheetmetal such as steel identified by the number 250. It has a rectangularaperture that is congruent with aperture 152 in FIG. 15. Plate 250 alsohas a planar margin 252 and an upstanding margin 253 which fits into acomplementarily shaped groove in the track for the masks and preventssidewise shifting, as described earlier. The next layer is a sheet oflead 254 which also has a congruent rectangular aperture. Bonded to topsurface of lead sheet 254 is another thinner steel sheet 256 which hasan aperture that is congruent with the others. Finally, top layer is alead sheet 258 which has lesser external dimensions than the sheets orplates on which it is superimposed, but it has an aperture of similarshape. Metal sheets, 250, 254, 256, and 258 are bonded at theirinterfaces with a rigid epoxy resin to form a unitary mask. Bonding twostiff metal layers to opposite sides of a soft metal layer results in abeam which has the soft layer lying on the neutral bending axis and thestiff layers spaced therefrom so the modulus of the beam is high.

Substantially, the same stiffening effect may be obtained if the sheetsare stacked in a different sequence. For example, the soft lead andhighly x-ray absorbent sheet 254 could be bonded to the bottom of steelsheet 250 and steel sheet 256 could be bonded to the bottom of it whilenarrower lead sheet 258 could be bonded to its topsheet 252.

Note that the width of the sheets or plates comprising the mask becomenarrower progressing upwardly. The wide lead sheet 254 intercepts mostof the stray radiation laying outside of the collimated beam which comesup from the x-ray source. The next steel sheet layer 256 is narrowersince it is employed primarily for stiffening the mask in its centralsection where the lead sheet needs it most to avoid sagging. The marginof the top lead sheet 258 is quite narrow and only has sufficient widthfor intercepting any radiation that might result from inacurracy in thesetting of the shutters of the collimator, not shown, which isassociated with the x-ray source in the table body.

As mentioned earlier, the spot film device is designed for permittingloading and unloading of a cassette from the front or from the rear. Asafety feature for rear loading will now be described in reference toFIGS. 2 and 3. FIG. 2 shows a plan view of the spot film device 17 withthe fluoroscopic device 20 removed. Ordinarily, the fluoroscopic deviceis coupled to an apertured plate 265. This view also shows the handle266 which may be used by the radiologist to cause the spot film deviceto be driven lengthwise of the x-ray table body. Another handle 267 isprovided for manipulating the spot film device as required when thex-ray table is tilted to place the patient in an upright position.

In FIG. 2, the opening for inserting and withdrawing a cassette from therear of the spot film device is marked 268. The opening must be enteredto actuate an operating lever for lifting the cassette out of the tray.This is a simple lever which is shown in FIG. 8, where it is marked 269.It has a flat extension 270 which causes the cassette to be lifted whena handle 271 is grasped for pivoting the lever. Also shown in FIG. 8 isa detent device 272 which holds the cassette in place. The detent has aplunger 273 acted on by a spring 274 which is captured by a cap 275. Thetip 276 of the plunger 273 is beveled as a two sided wedge so that whena cassette frame is pushed into cassette tray 95, the beveled tip willrecede and then pass over the cassette to secure it in place under theinfluence of spring 274. A sufficient upward force on the cassettedeveloped manually with lifting lever 269 will overcome the force ofspring 274 and allow the cassette to be removed.

Referring to FIG. 2, one may see that the opening 268 has tape switches277 and 277' at its margins. One of the tape switches is shown insection in FIG. 3. It comprises a flexible rubber cover 278 whichsupports a strip 280 of a conductive material such as spring brass whichhas a concave cross section. The metal strip 280 constitutes an electriccontact. There is another metal strip 280' mounted on rubber 282 whichserves as another contact. Normally, metal contact strips 280 and 280'are electrically isolated by thin insulator strips 281 and 281' and byan air space 283. A thick sponge strip 284 supports the tape switch. Thesponge affords further resilience. When a force is applied to rubbercover 278, such as when an operator's hand is inserted through thecassette opening or hatch 268, the cover deforms and causes metal strip280 to make electric contact with metal strip 280'. By closing a circuitin this manner, a relay device, not shown, may be operated to causedriving circuits to all motors to be opened so that no mechanism can bemoved or operated when a hand is in contact with an edge of the cassetteopening.

FIG. 17, comprised of parts 17A - 17E, shows various ways in which thefilm cassette may be shifted relative to the masks to expose differentareas of the film and to make exposures of different sizes.

In FIGS. 17A and 17B, the mask is advanced the same radiographicposition for exposing the rear and front halves of the film. The film isoutlined in dashed lines marked 285. In FIG. 17A, the film projectsforwardly of the mask aperture so the rear half of the film can beexposed. In FIG. 17B, the film is shifted rearwardly for exposure of itsfront half. The two exposures are made by positioning the main carriageforwardly and then taking a step rearwardly.

FIGS. 17C and 17D show how two lengthwise halves of the film may beexposed sequentially by exposing the right half as in 17C and thenshifting the film by moving the inner cassette to expose the left halfas in FIG. 17D.

In FIG. 17E, the masks shown in 17B and 17D are superimposed so thattheir rectangular apertures are cross wise and in the example which isshown in 17E, the film is positioned for exposing the upper leftquadrant or corner area. As is well known, the film may be shifted threemore times to expose its remaining three quarter-sections.

For fluoroscopy, the masks are retracted from the x-ray beam but theindependently movable grid 154 may or may not be inserted in the beam.

A block diagram of the servo system for positioning the main carriage 38is shown in FIG. 18. A similar system is used for driving the innercarriage 42. In FIG. 18, the main carriage motor is marked 55 and thepotentiometer for sensing the position of the carrige is marked 66,consistent with their markings in FIG. 4. Motor 55 is driven from theoutput of a power amplifier 300 which is supplied from an operationalamplifier 301. An input to amplifier 301 connects to a summing junction302. A signal indicative of the instantaneous position of carriage 38 issupplied to junction 302 from the output of a differential amplifier303. The signal corresponding with carriage position is supplied toamplifier 303 from position sensing potentiometer 66.

Any desired position of the carriage is obtainable by grounding one ofseveral input terminals such as 304 and 305. A different signal issupplied to summing junction 302 for each carriage position desired. Apotentiometer 306 is used to develop a signal that corresponds with onedesired position of the carriage, for instance, its most advancedposition. The wiper of potentiometer 306 is connected through a resistor307 to field effect transistor 308. The output of transistor 308connects to summing junction 302. When gate 309 is grounded, a signalcorresponding with the desired position is delivered to junction 302 andthe carriage is moved until the signal from position sensingpotentiometer 66 balances it and motor 55 stops driving. Another typicalposition is the parked position of the carriage which is obtained bygrounding input terminal 305. It similarly turns on a field effecttransistor 310 which conducts a corresponding position signal from apotentiometer 311 which has a resistor 312 between its wiper and thetransistor.

A control circuit, typical of that used for the motors that drive themasks 147 and 151 and the grid 154 is shown in FIG. 19. One of the maskdrive motors 175 is used for an example. Motor 175 is a reversible d-cmotor. It is mechanically coupled to mask 151 as symbolized by thedashed line 320.

The control circuit comprises a relay coil K1 connected between a d-csource terminal V and ground G. The coil has a switch SW3 in series withit. As shown in FIG. 19, when switch SW3 is open, the mask 151 is drivento its parked position. When the switch is closed, the mask is driven toradiographic position. Relay K1 controls contacts K1A, K1B, K2A and K2B.The state of the contacts as shown in FIG. 19 corresponds with the maskbeing in parked position. If switch SW3 is now closed, coil K1 isenergized and contacts K1A and K1B open. K2A and K2B close. Thisprovides a circuit for driving motor 175 in one direction. Current flowunder these conditions is from terminal V through a series circuit inthe sequence of contact K2A, diode D1, limit switch SW2, motor 175, andcontact K2B to ground G. When the mask reaches radiographic position, itoperates a plunger in the limit switch assembly 186 in FIG. 10, whichplunger is marked 321 in FIG. 19. Actuation of plunger 321 results inlimit switch SW2 opening to de-energize the motor. However, sincecontact K2A has remained closed, a resistor R1 is connected in serieswith motor 175 for reducing its power as compared with the power appliedto the motor before plunger 321 is struck. Thus, when mask 151 stops inits final position against stop 189, reduced power is maintained on themotor.

Motor 175 is caused to drive mask 151 to parked position by openingswitch SW3. The contacts then assume the states in which they are shownin FIG. 19. In this case, current flows from supply terminal V throughcontact 1A, motor 175, diode D2, limit switch SW1 and contact K1B toground. This direction of current flow results in motor 175 operating inthe reverse from the preceding example. When the mask reaches parkedposition it strikes a plunger 322 before hitting stop 198 and limitswitch SW1 opens. Since contacts K1A and K1B remain closed, reducedpower is again applied to motor 175 by reasons of resistor R1 being inseries with it between terminal V and ground.

We claim:
 1. For use with a diagnostic x-ray table including a tablebody, an x-ray source in the body and a table top for supporting anexamination subject in the path of an x-ray beam from the source, a spotfilm device adapted to be located on a side of the table top remote fromthe source, said spot film device comprising:support means for extendingcrosswise of said table top, carriage means mounted for translating onsaid support means between an inactive position and a radiographicposition, means on said carriage means for holding a cassette, agenerally planar first x-ray grid means mounted to said carriage meansfor being interposed between said x-ray source and a cassette when saidcarriage means is advanced to radiographic position, a second x-ray gridmeans mounted for movement on said support means between a retractedposition and an advanced position where said second grid means may beused alone for fluoroscopy or jointly with said first grid means forradiography, said second grid means being disposed orthogonally relativeto said first grid means, means for accommodating a fluoroscopic devicein alignment with the path of said beam from said x-ray source, meansincluding reversible motor means and means operatively coupling saidmotor means to said second grid means for selectively advancing saidsecond grid means alone to perform fluoroscopy or jointly with saidcarriage and said first grid means to perform radiography andalternately maintaining said second grid means and carriage in retractedposition to permit fluoroscopy without any grid means in said x-ray beampath, drive pulley means driven by said reversible motor means, idlerpulley means spaced from said drive pulley means, stop means forlimiting travel of said second grid means, said means for coupling saidmotor means to said second grid means comprising belt means running onsaid idler and drive pulley means, said belt means having opposite endportions connected to said second grid means and said belt means havingan elastic characteristic for enabling said belt means to yield andabsorb shock and allow overtravel by said motor means when said secondgrid means is stopped abruptly by said stop means, and means respondingto said second grid means reaching a position near said stop means byreducing the electric power supplied to said motor means and bymaintaining said reduced power for holding said grid means againt saidstop means.
 2. The device as in claim 1 wherein spring means areinterposed between each of said opposite ends of said belt means andsaid second grid means for obtaining said elastic characteristic.
 3. Foruse with a diagnositic x-ray table including a table body, an x-raysource in the body and a table top for supporting an examination subjectin the path of an x-ray beam from the source, a spot film device adaptedto be located on a side of the table top remote from the source, saidspot film device comprising:support means for extending crosswise ofsaid table top, carriage means mounted for translating on said supportmeans between an inactive position and an active radiographic position,means for holding a radiographic cassette on said carriage means, maskmeans mounted on said support means for being translated crosswise ofsaid table top selectively into and out of the path of the beam fromsaid x-ray source, said mask means having an aperture for defining thebeam impinging on a cassette in said means for holding a cassette,reversible motor means, drive pulley means driven by said motor means,idler pulley means spaced from said drive pulley means, belt meanstranslating said mask means and running on said drive pulley and idlerpulley means, said belt means having opposite end portions connected tosaid mask means, stop means for limiting the movement of said maskmeans, said belt means having an elastic characteristic for enablingsaid belt means to yield and absorb shock and allow overtravel by saidmotor means when said mask means is stopped abruptly by said stop means,means responding to said mask means reaching a position near said stopmeans by reducing the electric power supplied to said servo motor meansand then maintaining said reduced power for holding said mask meansagainst said stop means.
 4. The drive as in claim 3 including springmeans for obtaining said elastic characteristic, said spring means beinginterposed between each of said opposite end portions of said belt meansand said mask means.
 5. The device as in claim 3 including:track meansfor said mask means, and said mask means comprises a substantiallyplanar member having said aperture therein, said planar member havingopposite first and second margins which run on said track means, atleast said second margin being at an angle relative to said planarmember, said track means comprising first and second track members thatare spaced apart and parallel to each other, said first track memberhaving a groove which is shaped complementarily to said first margin toenable said first margin to slide in said groove and said second trackmember having a groove which is shaped complementarily to said secondangular margin to constrain said mask means against shifting sidewayswhile moving crosswise of said table top.
 6. The mask means as in claim5 wherein said angle of said second margin is substantially 90° relativeto the plane of said planar member.
 7. For use with a diagnostic x-raytable including a table body, an x-ray source in said body and a tabletop for supporting an examination subject in the path of an x-ray beamfrom said source, a spot film device adapted to be located on a side ofsaid table top remote from said source, said spot film devicecomprising:support means for being disposed crosswise of said table topand extending generally from the rear toward the front of said table,main carriage means and means for guiding said carriage means totranslate between a retracted position at the rear of said support meansand an advanced radiographic position at the front of said supportmeans, inner second carriage means and means on said main carriage meansfor guiding said second carriage means to translate in opposeddirections relative to said main carriage means and generallyperpendicular to the line of movement of said main carriage means,cassette holder means mounted to said second carriage means fortranslation therewith, means for translating said first carriage means,means for translating said second carriage means, individual generallyplanar mask means and means for guiding said mask means individuallybetween retracted position and advanced radiographic position alongparallel paths disposed between said cassette holder means and saidx-ray source, each of said mask means having an aperture for separatelyand cooperatively defining the x-ray beam on a cassette in said holdermeans, reversible motor means for the respective mask means andoperative to alternately advance said mask means to radiographicposition in the path of the beam from said x-ray source and to retractsaid mask means, pulley means disposed in spaced relationship along therespective paths of movement of said mask means, one of said pulleymeans being driven by said respective motor means, belt means running onsaid pulley means and coupled with one of said mask means and includingelastic means, stop means for stopping said mask means in advancedradiographic position, means responding to said mask means beingadvanced to a position near said stop means by causing a reduction inthe electric power supplied to said motor means such that said maskmeans will butt said stop means, said elastic means will be stressed toabsorb inertial force and said mask means will be held against said stopmeans under the influence of said motor means with reduced power appliedthereto, first anti-scatter grid means oriented in one direction andsecured on said main carriage means for being advanced to radiographicposition therewith, second anti-scatter grid means and guide means forguiding the same alternately between retracted position and jointradiographic and fluoroscopic position, driving means includingreversible motor means operatively coupled to said second grid means andoperative to advance and retract said second grid means, means fordisposing a fluoroscopic device in the path of said x-ray beam on a sideof said carriage means which is opposite from said source, said secondgrid means being advanced and retracted for permitting fluoroscopy,respectively, with and without said grid means in said beam and beingadvanceable with said first grid means for permitting cross gridradiography.
 8. The spot film device as in claim 7 wherein said beltmeans is a loop having opposite ends and said elastic means comprisesprings means coupled between the ends of said belt means and acorresponding mask means.
 9. The spot film device as in claim 7wherein:said driving means for said second grid means includes pulleymeans spaced from each other along the path of movement of said secondgrid means, said motor means being coupled in driving relation to one ofsaid pulley means, belt means running over each of said pulley means andattached to said second grid means, said belt means including elasticmeans, stop means for arresting movement of said second grid means,means responsive to said second grid means reaching a position near stopmeans by causing a reduction in the electric power supplied to saidmotor means such that said second grid means will butt said stop means,said elastic means will be stressed to absorb inertial force and saidsecond grid means will be held against said stop means under theinfluence of said servo motor means with reduced power applied thereto.10. The spot film device as in claim 9 wherein said belt means is anopen loop having opposite ends and said elastic means comprise springmeans interposed between each of said opposite ends and said second gridmeans.